The kinetic patterns of murine hematopoietic stem cell proliferation will be investigated in vivo. The importance of hematopoietic stroma in sustaining blood formation in experimentally pancytopenic mice will also be studied. The administration of 89Sr, a bone-seeking, beta emitting isotope, in doses of 6 uc/gm or more, produces marrow aplasia, after which mice are sustained by splenic hematopoiesis. The stem cell compartment in these animals has thereby been concentrated in the spleen. 9/10 of the spleen in such animals can be removed surgically in a manner designed to retain splenic circulation to the remaining splenic remnant. We propose to follow the pattern of replication of primitive cells by the McCulloch-Till assay and by BFUe and CFUe culture assay after such a sudden contraction of the stem cell pool. The effectiveness of bone marrow, spleen, and embryonic liver fragments implanted in the peritoneum of splenectomized, 89Sr treated mice will be monitored in an effort to correct the thrombocytopenia, leukemia, and anemia of the hosts. Finally, the sensitivity of WWv and S1S1d mice to 89Sr and the effects of combinations of hematopoietic stroma, stem cells, and androgens will be compared with the results obtained by the same experimental manipulations in normal animals.